ULTRAVIOLET ABSORPTION SPECTRA l(i7 



where h = 6.61 X 10~" erg-sec 



h = 4.13 X 10-'-^ ev-sec 



c = 3 X 10'" cm/sec. 

 The energy differences corresponding to absorption bands in the ultra- 

 violet region (wave length <4000 A) are of the magnitude of 3.1 ev 

 or greater. Since these energies are greater than those that corre- 

 spond to the energy of formation of many chemical bonds (C — C bond 

 energy = 2.54 ev; C— N bond energy = 2.11 ev) (Pauling, 1945), the 

 rupture of such bonds in molecules raised to an excited level by absorp- 

 tion of an ultraviolet photon is energetically possible. Such rupture may 

 lead to the formation of free radicals or of oppositely charged groups or. 

 in molecules containing atoms with unbonded electron pairs, to photo- 

 oxidation and semiquinone formation (Waters, 1948; Lewis and Lipkin, 

 1942; Lewis and Bigeleisen, 1943b). The farther into the ultraviolet the 

 absorption band is located, the greater is the excess of excitation energy 

 over the minimum necessary for bond rupture. With absorption bands 

 in the far ultraviolet (wave length <2000 A) the absorbed energies gen- 

 erally become adequate to produce molecular ionization (8-12 ev) 

 (Price, 1947). 



WIDTH OF ABSORPTION BAND 



The width of an individual absorption band is dependent intramolecu- 

 larly on the duration of the excited electronic state (Heitler, 1944, pp. 

 110#) and extramolecularly on the statistical distribution of the fre- 

 quencies of the particular absorption band among the assemblage of 

 absorbing molecules, each exposed to a certain randomness of molecular 

 environment. 



Considering any one molecule in a given molecular environment, the 

 width of its absorption band is inversely dependent on the duration 

 (mean lifetime) of the excited electronic state. This may be formulated 

 by the "uncertainty principle" 



AE At ^ ^ 



ZTT 



where, in this instance, AE is the uncertainty, i.e., variation, in the energy 

 difference accompanying the transition and A^ is the duration of the 

 transition. For the usual absorptive process in an isolated mole- 

 cule, At is of the order of magnitude of IQ-^ sec, A^ is about 10"' ev, 

 AE/E = 10-^ per cent as is AX/X. 



However, any of several processes may shorten the duration of the 

 excited state (A^, thus increasing the uncertainty in energy of the transi- 

 tion {AE), and hence may broaden the absorption band. Disruption of 

 the molecule may take place within the duration of a single moleculai' 

 vibration and thus reduce the excitation lifetime to as short as 10"'^ sec. 



